Welcome to Lemmy! Here's a simple guide for those migrating.
Lemmy is pretty much Reddit, with one major difference—it has instances. Imagine you could take all of Reddit's backend server code, copy and paste it, and host your own version of Reddit. Cool! Now you're an admin of Reddit 2. If the CEO of Reddit 1 ever does something mind-numbingly stupid, you and your users will be unaffected*. Reddit 1 and Reddit 2 are akin to Lemmy instances. Each instance has their own users and communities (subreddits).
Lemmy instances all work with each other*. Think of it like email. If I'm a Gmail user, I can still email Yahoo addresses, because Gmail, Yahoo, and all other email providers use a shared set of rules. All Lemmy instances also follow a shared set of rules (called a protocol, not that you need to know that). That means you can post to other instances, subscribe to their communities, upvote, downvote, and do all the things you can do on your instance.
The Lemmy app you use has a lot of control over how this is all laid out, so if you're finding it confusing, try different apps. As a former RIF user, I personally like Liftoff, which you can get in the Play Store, but other popular options are Jerboa, Thunder, Connect for Lemmy, wefwef, and Summit.
There's plenty more to say, but that should be enough to get started. Feel free to post questions in the comments! I may not know the answers to all of them, but I imagine others will.
*Oversimplification.